Big hole cutter

ABSTRACT

A big hole cutter such as are employed in connection with raise drilling machines and box hole drilling machines in which a support frame is provided for connection to the drill string of the machine and on which support frame there is mounted rotary cutters on the side facing the formation to be reduced. The frame is a skeleton-like structure in that openings are provided extending axially therethrough which the muck and debris taken by the cutters can fall thereby preventing the cutter from becoming clogged up in operation. The big hole cutters for either raise drilling or box hole drill operations can include stabilizer rollers about the periphery to engage the surface of the hole being drilled and thereby stabilize the big hole cutter in the lateral direction. With raise drilling cutters, the actuating drill string connects to the cutter at the top and the raise drilling cutter is pulled upwardly while in a box hole machine the drill string connects to the bottom of the big hole cutter and the cutter is pushed upwardly. In respect of the big hole cutters for the last mentioned type of machine, the cutter also includes a pilot cutter extending axially from the upper end thereof and the aforementioned stabilizing rollers are particularly important for guiding the big hole cutter during working operations.

United States Patent i 1 l l l l 3,858,667

Goodfellow l l Jan; 7, 1975 [5 BIG HOLE CUTTER 57] ABSTRACT i 1 Inventor? Robe" Goodfellow, Bedfordi A big hole cutter such as are employed in connection [73] Assigneez Kennametal Inc. Latrobe pal; with raise drilling machines and box hole drilling machlnes ln whlch a support frame'is provlded for con- [22] Flled: 1973 nection to the drill string of the machine and on which {21] APPL 411,743 supportframe there mounted rotary cutters on the slde facing the formatlon to be reduced. The frame ls a skeleton-like structure in that openings are provided [52] US. Cl 175/53, l75/345, 175/355, extending axially thgrethmugh which h muck and 175/375 debris taken by the cutters can fall thereby preventing [51] Int. Cl. Elle 23/00 the Cutter f hgcommg clogged up in opcratiom [58] Field of Search 175/53 The big hole cutters for either raise drilling or box 175/353, 355, 357, 361i 363, 375 hole drill operations can include stabilizer rollers aboutthe periphery to engage the surface of the hole 1 References Cited being drilled and thereby stabilize the big hole cutter UNITED STATES PATENTS in the lateral direction. With raise drilling cutters, the

25 2 |2/|953 Merrick [75/53 X actuating drill string connects to the cutter at the top 2,823,900 2/l958 Kandle 0 175/53 X and the raise drilling cutter is pulled upwardly while in 3,0ll,567 12/1961 Turner 175/53 a box hole machine the drill string connects to the 3,231,029 1/1966 Winberg 175/53 bottom of the big hole cutter and the cutter is pushed 175/344 X upwardly. In respect of the big hole cutters for the last 3,750,767 8/1973 P855181 175/53 mentioned yp of machina the Cutter also includes a FOREIGN PATENTS OR APPLICATIONS pilot cutter extending axially from the upper and 391858 12/1958 Germany 175/53 thereof and the aforementioned stabilizing rollers are Primary Examiner-David H. Brown Attorney, Agent, or FirmMelvin A. Crosby particularly important for guiding the big hole cutter during working operations.

PATENTEDJA" H915 7 3,858.66?

sum 10F 6 FIE. 1

PATENTEU JAN 71975 3.858.667, FIE. 4

SHEET 4 OF 6 FIE.E

PATENIED JAN 7 i975 saw 5 or 6 BIG HOLE CUTTER The present invention relates to big hole cutters such as are employed in connection with raise drilling ma chines and box hole drilling machines and is particularly concerned with the construction of the frame which supports the individual rolling cutters of the big hole cutter. I

Big hole cutters of the type with which thepresent invention is concerned are well known and comprise frame structures which are circular when viewed in plan and which have distributed over one side thereof rolling cutters having their axes extending substantially radially of the frame. The rolling cutters are distributed radially and circumferentially on the frame and when the frame is moved in the axial direction while being rotated, the rolling cutters engage the formation and reduce the formation principally by crushing and cutting the formation. I

Inasmuch as the big hole cutter is advancing into the formation, there is a tendency for the material, or muck, taken from the formation by the rotary cutters to become entrapped between the face of the forma tion and the frame of the cutter. This causescomplications and prevents the rolling cutters from working efficiently because they do not have clean contact with the formation being reduced.

With the foregoing in mind, a particular object of the present invention is the provision of a big hole cutter of the nature referred to so constructed and arranged that no problems occur on account of debris, or muck, being entrapped between the frame of the big hole cutter and the face of the formation being reduced.

Another object of the present invention is the provision of a frame or base plate assembly for a big ho|e cutter including openings extending axially therethrough for the passage of debris taken from a formation by rolling cutters mounted on the base plate.

A still further object is the provision of'a frame or base plate for a big hole cutter according to the preceding object in which the strength of the frame or base plate is extremely high and is in no way diminished by the passages extending axially therethrough for'the debris.

BRIEF SUMMARY OF THE INVENTION rection to bring the rolling cutters into pressure engagement with the face of a formation to be reduced and by rotating the big hole cutter while maintaining the pressure the formation is reduced. 7

It is usually the case that big hole cutters rotate on a generally vertical axis and move in the upward direction. Because of this, debris, or muck, taken from the formation tends to accumulate on the upper side of the frame structure and to interfere with the engagement of the rolling cutters witn the formation.

The present invention arranges the base structure from concentric rings interconnected by spoke-like structures and support plates so that between the inner and outer rings there are provided axial spaces through which the debris taken by the rolling cutters can fall.

The rolling cutters are mounted on the spoke-like structures and on other support arrangements provided on the outer ring of the frame structure. The spoke-like structures are made in the form of hollow beams which are reinforced so that the large torques that are encountered for rotating the big hole cutters can be transmitted through the frame structure from a drill string connected to the frame structure on the axis to the rolling cutters which are mounted on the frame structure in distributed relation.

The nature of the present invention and the various objectives and advantages thereof will become more clearly apparent upon reference to the following detailed specification taken in connection with the accompanying drawings in which:

FIG. 1 is a plan view looking down on top of a big hole cutteraccording to the present invention and especially adapted for use with a raise boring machine.

FIG. 2 is a sectional view of the big hole cutter of FIG. 1 and is indicated by section line II-II on FIG. I.

FIG. 3 is a plan view of the frame structure of the big hole cutter of FIGS. 1 and2 showing the arrangement thereon of the support areas for the rolling cutters to be mounted on the frame structure.

FIG. 4 is an irregular sectional view indicated by line IV,-IV on FIG. 3 and showing more in detail two of the regions for receiving rolling cutters and also showing one of the passages extending axially completely through the frame structure.

FIG. 5 is a fragmentary sectional view indicated by line VV on FIG. 3 and showing another ofthe rolling cutter supports of the frame structure.

FIG. 6 is a fragmentary side elevational view of the frame structure showing a typical support for a rolling cutter and the manner in which it is braced on the frame structure.

FIG. 7 is a view similar to FIG. 2 and is a vertical section through a bigv hole cutter according to the present invention especially constructed and arranged for use with a box hole drilling machine.

FIG. 8 is a plan sectional view indicated by line VIII- VIII on FIG. 7 and illustrating somewhat more in detail the placement on the frame structure of the stabilizing rollers forming a part thereof.

DETAILED DESCRIPTION OF THE INVENTION Referring to the drawings more in detail and, in particular, to FIGS. 1. 2 and 3. the big hole cutter according to the present invention is made upof a base frame or base structure generally indicated at 10 and formed of a tubular inner part 12 and a tubular outer part 14 concentric with the inner part.

Inner part 12 may consist, for example, of steel tub ing with a wall thickness of about one and A inchand with about 12% inches outside diameter. Outer tubular member 14 may be a steel tube having a wall thickness of about 1% inches and about 33 inches outside diameter. The sizes given are those for about a 5 foot cutter. In length, the tubular members could be on the order of about l2 inches.

Mounted in the inner tubular member 12 is a member 16 which extends upwardly from the base frame on the axis thereof and whichhas a threaded upper end 18 for connection to the lower end ofdrill string by means of which the big hole cutter is rotated while being pulled upwardly in the axial direction.

Mounted in distributed relation on the upper side of the base frame are rolling cutters. The rolling cutters are substantially identical except for length and the angle of inclination to the axis of the big hole cutter. Each rolling cutter has a saddle portion fixed to the base structure of the big hole cutter and a body part rotatably supported in the saddle.

The rolling cutters consist of a first pair of rolling cutters 20 and 22 nearest the axis of rotation of thebig hole cutter and having their axes of rotation substantially perpendicular to the axis of the big hole cutter. Each rolling cutter has a respective saddle portion 24,

26 and rotatably mounted on a shaft fixed to the saddle structure is a respective rotatable body part 28. 30. Each body part tapers inwardly toward the axis of rotation of the big hole cutter and is preferably studded with hard wear resistant inserts such as rods of cemented tungsten carbide having rounded outer ends.

Outwardly from rolling cutters 20 and 22 is a further pair of rolling cutters 32, 34 which are supported with the axes of rotation at such an angle that the outer surfaces of the respective rotatable bodies thereof incline downwardly in a direction away from the axis of rotation of the big hole cutter at an angle of'about Still further outwardly from cutters 32 and 34 is a'further pair of cutters 36, 38 having their axes of rotation so inclined that the peripheries of the rotatable body parts thereof incline outwardly from the axis of rotation of the big hole cutter at angles of about 21 degrees. The aforementioned cutters are arranged in diametrally opposed pairs as will beseen from FIG. 1.

Still further outwardly on the base structure of the big hole cutter is a group of rolling cutters identified at 40, 42, 44 and 46, each of which has theaxis of rotation inclined outwardly so that the peripheral surface of the rolling body of each cutter, on the side facing the formation, inclines outwardly from the axis of rotation of the big hole cutter at an angle of about 27.

The rotating bodies of the respective rolling cutters substantially overlap as will be seen in FIG. 2 so that, when the big hole cutteris viewed from the side, a substantially continuous cutting surface is provided which is somewhat concave upwardly and the outermost limit of which cutting surface is disposed radially outwardly from the radial outer limits of the base structure.

By way of example, the diameter on which rolling cutters and 22 are disposed may be displaced about 80 from the diameter on which rolling cutters 32 and 34 are disposed and about 38 degrees from the diameter on which rolling cutters 36 and 38 are disposed. The diameter on which rolling cutters 40 and 42 are dis posed may be displaced about 20 from the diameter of rolling cutters'20 and 22 and 90 from the diameter on which rolling cutters 44 and 46 are disposed.

The manner in which the base structure is made will be clearly seen from FIGS. 3 through 6. In FIGS. 3 to 6, it'will be noted that the inner portion 12 is connected to the outer tubular portion 14 by spoke-like elements and which spoke-like elements may consist of a pair of diametrally opposed axially extending plates 50 and 52 welded in place between the tubular portions and a plurality of beam-like structures 54 distributed in the space between the tubular members and provided for supporting rolling cutters 20, 22, 32 and 34.

Each of the beam-like structures 54 comprises a pair of parallel plates 56 and 58 extending axially between members 12 and I4 and welded thereto and a further plate 60 extending diagonally between members 12 and 14 in the space between plates 56 and 58 and also welded in position.

A top plate 62 and a bottom plate 64 is provided for each beam-like structure and is welded in place in the top and bottom thereof. The beam-like structures impart great rigidity to the connection established thereby between inner tubular member I2 and outer tubular member 14 and, likewise.-provides solid supports for the saddles of the rolling cutter'assemblies which are mounted thereon.

For further increasing the strength of the structure, triangular gusset plates 65 are welded in place between the beam-like structures 54 and the inside of tubular member 14 near the upper and lower ends ofthe beamlike structures.

For supporting the saddles pertaining to rolling cutters 36'and 38'which are disposed outwardly of the outer periphery ofouter tubular member I4, support arrangements such as is illustrated in FIG. 5 are cmployed. FIG. 5 shows that there is an inclined support plate 66 extending througha notch 68 formed in the upper edge of outer tubular member 14 and with the adjacent radial plate 52 also notched as at 70 to accommodate support plate 66.

Depending from opposite sides of support plate 66 and extending along the periphery of outer tubular member 14 and welded in place are bracing plates 72 and a further plate 74 extends along the outer edges of plates 72 perpendicularly to support plate 66 and also welded in place. Further to reinforce the support arrangement, gusset plates indicated at 76 can be provided, one near thetop of the support arrangement, and one near the, bottom and extending inwardly to the outer tubular member I4 and welded in place.

The support arrangement for supporting the outermost rolling cutters 40 to 46 is illustrated in section in FIG. 4 and in side elevation in FIG. 6. This support structure also comprises an upper support plate 80 on which the saddle of the respective rolling cutter rests, spaced side plates 82 interposed between support plate 80 and the outer periphery ofouter tubular member 14 and an outer plate 84 at the outer edges of plates 82 and perpendicular to support plate 80 and welded in place. Bracing gussets as indicated at 86 in FIG. 6 are also provided between the sides of the support members and the outer periphery of tubular member I4.

In respect of the support regions for the second group of rolling cutters, represented by cutters 32 and 34, the saddles thereof extend outwardly beyond the outer periphery of outer tubular member 14 at least a short distance and support is provided for the overhanging portions of the saddle members by gusset-like support plates 88 welded to the outer periphery of support member 14 and forming substantial continuations of the plates 56 and 58 on the inside of the tubular member I4 in the same circumferential region.

It will be appreciated that the base on frame structure described above provides extremely rigid and strong support for the rolling cutters mounted thereon and is so constructed and arranged by virtue of the boxlike beams extending radially between the inner and outer tubular portions to transmit extremely high torques from the drill stem connected to the big hole cutter through the base structure to the rolling cutters.

In respect of the conveying of torque, the base'frame generally turns in the clockwise direction when viewed from above during working operations and 'the inclined plates60 on the inside of the box beam structures become particularly effective during working operations for insuring transmission of torque from the inner tubular member 12 to the rolling cutters and also to the outer tubular member 14.

It has been mentioned that the base structure provides openings for debris to fall therethrough, and it will be seen, particularly in FIGS. 3 and 4, that, when viewed in plan, the base structure has a plurality of large openings extending axially therethrough, one such opening being identified by reference numeral 90 while in the circumferential region between adjacent ones of those rolling cutters which are disposed at least partly outwardly from the outer periphery of the outer tubular member 14, there is substantial clearance that will permit debris readily to fall therethrough.

The big hole cutter of the present invention is, thus, free of accumulation of debris on the top that would interfere with the proper action of the rolling cutters upon the formation being reduced. The big hole cutter, thus, operates more efficiently and at lower torque than would be the case if the construction of the big hole cutter were such that debris could accumulate on top thereof. At the same time, the axial passages for the debris to fall through have been obtained at no sacrifice in strength of the base structure of the big hole cutter.

It will be evident that, while the big hole cutter shown has a nominal size of, say, 5 feet, the same principles of construction could be utilized in extending the diameter of the big hole cutter to substantially any desired size while the base structure thereof would always have axial passages to permitthe passage of debris there through.

FIGS. 7 and 8 show a modification of the arrangement of the present invention especially adapted for a box hole machine in which the big hole cutter is mounted on the upper end ofa drill string and is pushed upwardly into a formation to be reduced. In FIGS. 7 and 8, the tool shown comprises an upper portion generally indicated at 100 which is substantially the same as the structure previously described with the exception that, instead of the drill string adaptor at the upper end, there is provided a known pilot cutter arrangement 102.

Bolted to the bottom of base structure 104 of the upper part 100 of the tool of FIGS. 7 and 8, by virtue of a ring 106 fixed to the base structure, is a stabilizer frame 108. At the extreme bottom of the stabilizer frame, there is ring means 110 for connection of the frame to the drill string which will support and drive the big hole cutter.

The stabilizer frame also comprises an inner tubular member 112 and an outer tubular member 114. Inasmuch as inner tubular member 112 is that member which is principally loaded in torsion and compression, it is formed to a greater thickness than outer member Furthermore, at the upper end member 112 is formed with a tenon which telescopes with the lower end'of inner member 116 of the base structure 104 of theupper part l00. Inner member 112 and outer member 114 are interconnected by circumferentially spaced axially extending plates 118 which are welded in place to form spoke-like elements and which support outer member 114 in concentric relation to member 112.

Outer member 114 is adapted for receiving stabilizer rollers 120 which are similar to rolling cutters except that the bodies thereof are preferably substantially cylindrical while also being studded with cemented hard carbide inserts. The purpose of the stabilizers is to support the big hole cutter against lateral deflection. This is important because the drill string is advanced by a drill string extending downwardly therefrom and which drill string is in compression and'can, therefore. deflect laterally. I

With the first described big hole cutter, the drill string was connected to the upper end thereof and was in tension and tended to deflect laterally a smaller amount while, furthermore, the drill string was substantially confined within a relatively small pilot hole through which the drill string extends downwardly to the big hole cutter.

With the modification of FIGS. 7 and 8, the drill string does not have the benefit of lateral support and for that reason the provision of stabilizer rollers 120 is of advantage. I

As will be seen in FIGS. 7 and 8, the stabilizer rollers are preferably connected to outer member 114 on suitable pad areas such as the plates 1 22 welded to the outside of outer member 114 and to which the saddles of the stabilizer rollers are connected. The stabilizer rollers are, in the modification illustrated, arranged in groups of three and at two levels, with the rollers in each group being uniformly circumferentially spaced and with one group being circumferentially offset from the other group as illustrated in FIG. 8,

The stabilizer roller arrangement of FIGS. 7 and 8 can be employed with the big hole raise cutter of FIGS. 1 to 6, but as explained, is not as critical a component of the big hole cutter as in connection with the box hole cutter of FIGS. 7 and 8.

In the case of either of the big hole cutter arrangements illustrated, any debris takenthereby as a cutting operation proceeds can readily fall through or around the base structure thereby maintaining the working area free of debris and thereby permitting the best possible cutting conditions to exist for efficient high speed operation of the big hole cutter under the smallest power requirement conditions.

Modifications may be made within the scope of the appended claims.

What is claimed is:

I. In a big hole cutter, especially for raise drilling and box hole drilling operations; a base structure adapted for rotation on a central axis, supports for rolling cutter devices mounted on the side of said base structure which faces the formation to be reduced and extending generally radially outwardly from said axis, and means on the axis of said base structure for connection of drill string means thereto for rotating said base structure while thrusting said base structure axially toward the side on which said rolling cutter supports are mounted, said base structure comprising inner and outer tubular members coaxial with the axis of rotation of said base structure, and substantially radial spoke elements disposed between and connected to said tubular members and circumferentially spaced to define axial channels through said base structure for the passage of debris taken by said rolling cutters, said spoke elements having the uppermost surfaces coplanar with the upper ends of the tubular members, at least some of said spoke elements being in the form of box beams made up of top and bottom and side plates and forming supports for rolling cutters.

2. A big hole cutter according to claim 1 which includes a reinforcing plate in each box beam extending from the axial corner on one side of the beam at one end to the axial corner on the other side of the beam atthe other end.

3. A big hole cutter according to claim 1 in which said box beams comprise one pair of beams on a first diameter of the base structure and a second pair of beams on a second diameter of the base structure which makes an angle different from a right angle with the first diameter, said spoke elements including radi ally extending axial plates between the most widely spaced ones of said box beams and connected at the radialends to said tubular members.

4. A big hole cutter according to claim 1 which includes a stem for connection to drill pipe fixed in said inner tubular member and extending axially from said base structure on the side toward said rolling cutter supports.

5. A big hole cutter according to claim 1 which includes a plurality of rolling cutters in radially and circumferentially distributed relation mounted on said supports therefor.

6. A big hole cutter according to claim I in which said box beams comprise a plurality of beams distributed circumferentially in the space between said tubular members, each box beam adapted for supporting a respective said rolling cutter.

7.'A big hole cutter according to claim 6 in which one diametral pair of said box beams is adapted for supporting a first pir of rolling cutters adjacent the axis of the base structure and another diametral pair of the beams is adapted for supporting a second pair of rolling cutters spaced radially outwardly from said first pair of rolling cutters. and means fixed to the outer periphery of said outer tubular member forming extensions of the upper surfaces of said other diametral pair of box beams.

8. A big hole cutter according to claim 1 in which said supports for rolling cutters include upwardly facing support plates, notches in the upper end of said outer tubular member through which said plates extend in a generally radial direction so as to protrude out- 8 wardly beyond the outer periphery of said outer member, and support bracket means beneath said plates connected thereto and to the outer periphery of said outer tubular member for supporting the said plates.

9. A big hole cutter according to claim 8 which includes lateral gusset plate means extending laterally between said support bracket'means and the outer periphery of said outer member to support said bracket means laterally.

10. A big hole cutter according to claim 8 in which said support plates are arranged respective radial distances from the axis of said base structure.

11. A big hole cutter according to claim I in which said tubular structure comprises a central tube and an outer tube and substantially radial circumferentially spaced axial plate elements extending therebetween and fixed thereto.

12. A big hole cutter according to claim 1] which includes means at the bottom of said tubular structure for connection thereof to drill pipe.

13. A big hole cutter according to claim 12 which in cludes means to mount a pilot cutter on the axis of said base structure on the side opposite said tubular structure.

14. In a big hole cutter, especially for raise drilling and box hole drilling operations; a base structure adapted for rotation on a central axis, supports for roll-' ing cutter devices mounted on the side of said base structure which facesthe formation tobe reduced and extending generally radially outwardly-from said axis. and means on the axis of said basestructure for connection of drill string means thereto for rotating said base structure while thrusting said base structure axially toward the side on which said rolling cutter supports are mounted, said base structure having axial channels there-through for the passage of debris taken by cutters on said supports, a tubular structure connected to the side of said base structure opposite said rolling cutter supports and coaxial. with the base structure, and circumferentially and axially distributed support means on the outside of said tubular structure for receiving stabilizer rollers having the radially outer most regions in substantial axial alignment with the radially outermost regions of the rolling cutters mounted in the radially outer positionson said base structure.

15. A big hole cutter according to claim 1 in which said base structure is in the form of'a weldment. 

1. In a big hole cutter, especially for raise drilling and box hole drilling operations; a base structure adapted for rotation on a central axis, supports for rolling cutter devices mounted on the side of said base structure which faces the formation to be reduced and extending generally radially outwardly from said axis, and means on the axis of said base structure for connection of drill string means thereto for rotating said base structure while thrusting said base structure axially toward the side on which said rolling cutter supports are mounted, said base structure comprising inner and outer tubular members coaxial with the axis of rotation of said base structure, and substantially radial spoke elements disposed between and connected to said tubular members and circumferentially spaced to define axial channels through said base structure for the passage of debris taken by said rolling cutters, said spoke elements having the uppermost surfaces coplanar with the upper ends of the tubular members, at least some of said spoke elements being in the form of box beams made up of top and bottom and side plates and forming supports for rolling cutters.
 2. A big hole cutter according to claim 1 which includes a reinforcing plate in each box beam extending from the axial corner on one side of the beam at one end to the axial corner on the other side of the beam at the other end.
 3. A big hole cutter according to claim 1 in which said box beams comprise one pair of beams on a first diameter of the base structure and a second pair of beams on a second diameter of the base structure which makes an angle different from a right angle with the first diameter, said spoke elements including radially extending axial plates between the most widely spaced ones of said box beams and connected at the radial ends to said tubular members.
 4. A big hole cutter according to claim 1 which includes a stem for connection to drill pipe fixed in said inner tubular member and extending axially from said base structure on the side toward said rolling cutter supports.
 5. A big hole cutter according to claim 1 which includes a plurality of rolling cutters in radially and circumferentially distributed relation mounted on said supports therefor.
 6. A big hole cutter according to claim 1 in which said box beams comprise a plurality of beams distributed circumferentially in the space between said tubular members, each box beam adapted for supporting a respective said rolling cutter.
 7. A big hole cutter according to claim 6 in which one diametral pair of said box beams is adapted for supporting a first pir of rolling cutters adjacent the axis of the base structure and another diametral pair of the beams is adapted for supporting a second pair of rolling cutters spaced radially outwardly from said first pair of rolling cutters, and means fixed to the outer periphery of said outer tubular member forming extensions of the upper surfaces of said other diametral pair of box beams.
 8. A big hole cutter according to claim 1 in which said supports for rolling cutters include upwardly facing support plates, notches in the upper end of said outer tubular member through which said plates extend in a generally radial direction so as to protrude outwardly beyond the outer periphery of said outer member, and support bracket means beneath said plates connected thereto and to the outer periphery of said outer tubular member for supporting the said plates.
 9. A big hole cutter according to claim 8 which includes lateral gusset plate means extending laterally between said support bracket means and the outer periphery of said outer member to support said bracket means laterally.
 10. A big hole cutter according to claim 8 in which said support plates are arranged respective radial distances from the axis of said base structure.
 11. A big hole cutter according to claim 1 in which said tubular structure comprises a central tube and an outer tube and substantially radial circumferentially spaced axial plate elements extending therebetween and fixed thereto.
 12. A big hole cutter according to claim 11 which includes means at the bottom of said tubular structure for connection thereof to drill pipe.
 13. A big hole cutter according to claim 12 which includes means to mount a pilot cutter on the axis of said base structure on the side opposite said tubular structure.
 14. In a big hole cutter, especially for raise drilling and box hole drilling operations; a base structure adapted for rotation on a central axis, supports for rolling cutter devices mounted on the side of said base structure which faces the formation to be reduced and extending generally radially outwardly from said axis, and means on the axis of said base structure for connection of drill string means thereto for rotating said base structure while thrusting said base structure axially toward the side on which said rolling cutter supports are mounted, said base structure having axial channels there-through for the passage of debris taken by cutters on said supports, a tubular structure connected to the side of said base structure opposite said rolling cutter supports and coaxial with the base structure, and circumferentially and axially distributed support means on the outside of said tubular structure for receiving stabilizer rollers having the radially outermost regions in substantial axial alignment with the radially outermost regions of the rolling cutters mounted in the radially outer positions on said base structure.
 15. A big hole cutter according to claim 1 in which said base structure is in the form of a weldment. 